Stray electrical currents associated with farm installations, particularly dairying equipment, can present a significant economic problem for farm operations. Dairy operations are susceptible to stray electricity because cows are extremely sensitive to electricity, much more so than humans, and will respond to potentials as low as one volt or less. The inherently moist environment of milking parlors and stalls further aggravates the stray voltage situation and makes contact between any stray electricity and the cows more probable. Dairy cattle affected by stray voltages will exhibit nervousness and a reluctance to enter the milking parlor, and may exhibit decreased milk production and increased levels of mastitis infection.
Stray voltage problems on farms usually have many causes. As a consequence, it is difficult and time consuming to detect all possible causes. A significant factor in excessive stray voltages on the secondary ground line is an inadequate grounding system. Such problems are described in a report by H. A. Cloud, et al "Stray Voltage Problems with Dairy Cows" North Central Regional Extension Publication 125. As noted in this report, in accordance with the National Electrical Code, neutrals and grounding conductors in the barn are bonded to the grounding terminal in the barn service entrance. The grounding terminal at the service entrance is bonded to the secondary neutral at the service transformer which in turn is bonded to the neutral line of the primary of the transformer. The purpose of the direct electrical connection between the primary or utility ground and the secondary or farm ground is to prevent the imposition of a very high voltage on the secondary lines if the transformer breaks down and a short circuit between the primary and secondary occurs. The connection between primary and secondary grounds will shunt the current back to the utility neutral, protecting the farm service lines from overvoltage and generally causing the utility fuses to interrupt current flow into the transformer. Since all parts of the grounded neutral network have some resistance to current flow, there can exist potential differences between portions of the neutral system and true ground. Potential differences in the power system's neutral are transmitted to the farm system by the electrical link between the primary and secondary neutral lines.
Several approaches have been proposed for blocking or isolating the voltage on the utility neutral from the secondary neutral. One approach involves the disconnection of the primary neutral from the secondary neutral at the distribution transformer.
This approach is the most practical and least expensive known to date, but merely opening the link can cause more problems than it solves. The ideal method of opening the link would include a means for closing it, very rapidly, in the event of trouble on the secondary system.
None of the approaches heretofore used or proposed are entirely satisfactory. For example, the installation of a spark gap or distribution arrester between the two neutrals has been suggested. Isolation of the primary and secondary neutrals in this manner may not, however, provide adequate protection against hazardous shocks to humans and animals or damage to equipment if a primary to secondary fault should develop within the transformer. Because of the potential danger, this approach is not acceptable to many power companies.
An alternative is the installation of a one-to-one isolation transformer on the secondary side of the main distribution transformer whereby the primary and secondary neutrals of the isolation transformer are electrically isolated from one another. In the event of a primary to secondary fault in the isolation transformer, the maximum voltage imposed on the secondary neutral would be 240 volts AC. Although such an approach is used, it has several drawbacks, of which cost is the most important.
Electronic switches have been proposed which will close swiftly when potential differences appear between the neutrals. For example, U.S. patent application No. 537,841 (group 214) , filed Sept. 30, 1983 by H. Tachick and E. Kotski (the present inventor), now abandoned, describes such a switch. Other switches are described in the references.
The drawback to these and other such electronic switches is their limited capability to "close in" on current sources which have high rates of rise and high magnitudes. They work properly when the source is limited in the amount of current it can deliver and in the rate of rise at which the current will be delivered. These switches will perform as intended in power supplies, inverters, and other such "normal" environments but be destroyed by a good size lightning bolt.
My invention is an electronic switch which will successfully close in on lightning surges of several tens of thousands of amperes having rise times in the order of 1 to 10 microseconds. This is about ten times the severity achieved by the present "state of the art" switches. How this is accomplished is described below.